Because of the prevalence of mobile terminals such as a cellular phone and a notebook computer, the role of a secondary battery used for their power sources has been increasing. The secondary battery is required to have the functions such as compactness, lightness, higher capacity and resistance to deterioration after the repetition of charging and discharging.
Metal lithium may be used as the anode of the secondary battery because of its higher energy density and lightness. However, in this case, with the progress of the cycles of the charging and the discharging, needle crystals (dendrite) deposited on the lithium surface during the charging penetrate a separator to cause the internal short-circuit failure, thereby reducing the battery life.
Lithium alloy having a composition formula of LiaA (A is a metal such as Al, and “a” satisfies 0<a≦5) has been investigated to be used as the anode. While the anode has a higher amount of inserting and extracting lithium ions per unit volume or a higher capacity, pulverization proceeds with the progress of the charging and discharging cycles due to expansion and contraction of the anode during the absorption and the desorption of the lithium ions. The pulverization causes a problem of the shorter charge-discharge cycle life.
If a carbon material such as graphite and hard carbon which is capable of inserting and extracting the lithium ion is used as the anode, a problem arises, though the charge-discharge cycles can be excellently repeated, that the energy density is lower because the capacity of the graphite material is lower than the metal lithium or the lithium alloy and the irreversible capacity of the hard carbon at the initial charging and discharging is large to make the charge-discharge efficiency lower.
Accordingly, a large number of investigations have been conducted to increase the capacity and the charge-discharge efficiency of the carbon anode.
JP-A-9(1997)-259868 discloses that aluminum, lead or silver having a smaller particle size acting as an auxiliary agent for charging and discharging the lithium ions is added to the carbon material to realize the higher capacity.
The PCT publication WO96/33519 discloses the use of amorphous metal oxide containing tin as the anode material. The amorphous metal oxide-anode is reported to repeat the charge-discharge cycles more excellently than the metal lithium or the lithium alloy.
JP-A-7(1995)-326342 discloses an anode for lithium secondary battery having a layered structure acting as an active material and prepared by using a carbon layer and a porous layer made of lithium alloy and formed on the carbon layer. The anode is reported to provide the lithium secondary battery having the larger discharging capacity together with the higher electromotive force.
For the purpose of preventing the deterioration of the anode, JP-A-5(1993)-234583 proposes the use of a carbon material coated with aluminum as the anode material, thereby preventing the intercalation of a lithium ion between carbon layers in the solvated state. As a result, the rapid deterioration of the cycle performance as well as the deterioration of the carbon layers can be suppressed.
On the other hand, JP-A-5(1993)-234583 discloses that a metal lithium foil affixed to the outermost section of an anode plate is allowed to diffuse into carbon for improving the charge-discharge efficiency. In accordance with this method, a secondary battery with a non-aqueous electrolyte which has higher energy density and is excellent in its anti-over-discharge performance can be obtained.
JP-A-5(1993)-234621 discloses a carbon material to which lithium powder acting as an anode active material is adhered in advance. A safety battery having the higher capacity can be provided by using the above anode because the difference between the capacities of the charge and the discharge can be eliminated.
JP-A-5(1993)-234621 also discloses a multi-layered electrode for a secondary battery in which an alkaline metal acting as an active material is supported on a carrier having carbon as a main component. Thereby, the anode for the secondary battery can be obtained which has a larger electrode capacity and is excellent in its charge-discharge cycle performance.
JP-A-5(1993)-242911 discloses the electric connection between a component, other than an anode active material, electrically connected to an anode and metal lithium in advance at the time of assembling the battery, thereby increasing the energy density and improving the over-discharge performance.
JP-A-10(1998)-144295 discloses an anode which is prepared by vapor-depositing an electro-conductive metal which is not alloyed with lithium on a carbon material surface followed by the evaporation of the lithium on the electro-conductive metal. In the anode, the lithium is efficiently inserted in the anode active material to securely compensate the capacity loss of the anode. Thereby, in addition to the increase of the initial charge-discharge efficiency, the battery capacity can be increased and the charge-discharge cycle performance can be improved.
JP-A-5(1993)-275077 discloses an anode for lithium secondary battery which is prepared by coating a carbon material acting as an anode component with a lithium-ion conductive solid electrolyte film. The improved lithium secondary battery can be provided which uses the carbon material as the anode and propylene carbonate as at least part of an organic solvent acting as electrolyte.
JP-A-2000-182602 discloses an anode for a secondary battery which is prepared by affixing a metal foil containing lithium as a main component on an anode sheet made of amorphous oxide which enables to insert and extract lithium.
However, the technique in which small-sized aluminum is added to a carbon material disclosed in JP-A-9(1997)-259868 arises problems such that the charge-discharge condition of an electrode is made non-uniform, and the deformation of the electrode and the peeling-off of an active material from a current collector are generated due to the local concentration of an electric field after the repetition of charge-discharge cycles because the uniform dispersion of the metal particles in the carbon material is difficult and the metal is localized in the anode. Accordingly, the cycle performance can be hardly maintained at a higher level.
An SnBbPcOd (“b” is 0.4 to 0.6, “c” is 0.6 to 0.4 and “d” is 1 to 7) metal oxide amorphous material disclosed in WO96/33519 has a problem that the energy density of a battery can be hardly increased to a sufficient level because the irreversible capacity after an initial charge-discharge is larger.
Further, the above prior arts have a common problem that a higher operating voltage cannot be obtained. This is because the operating voltage of the electrode prepared by mixing the metal with the carbon material is lower than that of the anode made of only the carbon since the plateau specific to the metal at a voltage higher than that of the carbon appears on its discharge curve. The lower limit voltage for a lithium secondary battery is prescribed depending on its use. Accordingly, the lower operating voltage narrows the usable region, and consequently the capacity increase in the region where the battery is actually used can be hardly attained.
On the other hand, the anode material containing aluminum and disclosed in JP-A-5(1993)-234583 has a problem of the rapid capacity decrease after the repetition of cycles. The reason thereof seems the formation of a thin insulation film on the aluminum surface generated through a reaction between an impurity such as water existing in an electrolyte and the aluminum.
In the methods disclosed in JP-A-5(1993)-144473, JP-A-5(1993)-234621, JP-A-5(1993)-242911, JP-A-5(1993)-275077 and JP-A-7(1995)-326342 in which the lithium metal or the lithium alloy is mixed with, or added to or affixed to the carbon anode, the improvement of the charge-discharge efficiency is insufficient. This is because, when the carbon is in direct contact with the metal Li or Li alloy, a film is formed on the carbon surface through a reaction between the added metal Li or Li alloy and impurities such as an active functional group on the carbon surface and adsorbed water. The lithium contained in such a film is electrochemically inactive and cannot contribute to the increase of the charge-discharge capacity of the battery. Accordingly, the above methods cannot sufficiently improve the charge-discharge efficiency. Further, the larger electric resistances of these films increase the resistance of the battery to decrease the effective capacity of the battery.
While JP-A-2000-182602 discloses the anode for the secondary battery prepared by affixing the metal foil containing the lithium as the main component on the anode sheet made of an amorphous oxide which enables to insert and extract the lithium, the specific disclosure with respect to the anode sheet made of the amorphous oxide includes active materials such as Sn, Al, B, P and Si solidified with a binder. The non-uniformity of the metal distribution can be microscopically inevitable in the above sheet, thereby generating the local concentration of the electric field.
The direct contact between the binder and the lithium metal foil generates a higher-resistance film by the reaction between the binder and part of the lithium metal foil. Due to the above reasons, the cycle performance can be hardly maintained at a higher level.
When a battery is configured to have the same charge-discharge capacities for a cathode and an anode as in an ordinary lithium ion secondary in which the charge-discharge capacity of the anode is larger, the reversible capacity or the capacity usable for the repetitive charge and discharge steps of the cathode is larger than that of the anode as shown in FIG. 1a. The reversible capacity of the battery is the same as that of the anode. That is, the difference (C1−A1) between the reversible capacity of the cathode C1 and the reversible capacity of the anode A1 does not contribute to the reversible capacity of the battery, thereby maintaining the energy efficiency to be lower.
In the lithium secondary battery, the charge and the discharge can be performed most efficiently when the reversible capacities of the cathode and the anode are equal to each other. When, upon considering this, a reversible capacity of the whole anode A2 is made equal to the reversible capacity of the cathode C by increasing the capacity of the anode by an amount of (C1−A1) as shown in FIG. 1b, the secondary battery excellent in its energy efficiency seems obtainable. However, in the secondary battery, the ratio itself between the reversible capacity and the irreversible capacity of the anode remains unchanged, and its irreversible capacity increases from B1 to B2.
Since the irreversible capacity part of the anode is at first compensated with the lithium component of the reversible capacity part of the cathode and then the reversible capacity part of the anode is charged, the energy efficiency of the secondary battery including the cathode and the anode having the capacities shown in FIG. 1b is not guaranteed to increase, and is likely to decrease. As described, the improvement of the energy efficiency in the method of relatively adjusting the capacities as shown in FIGS. 1a and b has its limit. It is most desirable that the reversible capacity of the anode is made equal or close to that of the cathode by increasing the ratio of the reversible capacity with respect to the irreversible capacity of the anode.